The selective action of certain antibiotics on various aspects of protein synthesis will be studied. (1) We will use purified polysomes, containing endogenous or viral natural messenger RNA, to pursue further two initial observations: that the misreading effect of streptomycin is exerted on already engaged polysomal ribosomes, and that various aminoglycosides differ in the ratio of their misreading effect to their inhibitory effect on such preparations. (2) Having found that spectinomycin and erythromycin, like streptomycin, block initiating ribosomes at some step after initiation we plan to identify the blocked step for each and possibly to extend this approach to additional antibiotics. These studies should advance our knowedge of the complex formation of the ribosome and should help lay the ground work for future studies of conformational differences between ribosomes blocked in different states by different antibiotics. Translation by ribosomes of natural polycistronic mRNA will be studied with purified, initiation-free polysomes enriched with specific types of messenger. Polypeptides formed from these polysomes will be isolated with specific antibodies made against them. N-terminal amino acid and the size of the peptides formed from these polysomes will be analyzed, and the results should enable us to determine whether ribosomes come off at the end of a gene or the end of a mRNA. The possible role of the ribosome release factor in this process will also be studied. A general search for conditional lethal mutants in translation will be initiated. These studies should advance our knowledge of how a functional protein or a hereditarily defective protein is synthesized and controlled. BIBLIOGRAPHIC REFERENCES: Tai, P-C, Impaired initiation complex formation on ribosomes treated with colicin E3. Biochem. Biophys. Res. Comm. 67:1466-1472 (1975). Tai, P-C. The interaction of streptomycin with ribosomes treated with colicin E3. (Abstract) Fed. Proc. (1976), in press.